 main{
     width: 600px;
     margin: 0 auto;
     border: 1px solid black;

 }
 header{
    height: 40px;
    background-color: black;

    
}
header>span{
    display: inline-block;
    width: 60px;
    height: 40px;
    text-align: center;
    color: white;
    line-height: 40px;
    margin-left: 40px;
}
.one{
    background-color: gray;
    color: black;
}
.two{
    display: block;

}
.three{
    display: none;
}